The more frequently a mother breastfed her child beyond the age of 24 months during the day, the greater the child's risk of severe early tooth decay, researchers found.
"The No. 1 priority for the breastfeeding mother is to make sure that her child is getting optimal nutrition," lead author Benjamin Chaffee of the University of California, San Francisco told Reuters Health.
Chaffee completed the study as a doctoral student at the University of California at Berkeley.
He and his team looked at a possible link between longer-term breastfeeding and the risk of tooth decay and cavities in a survey of 458 babies in low-income families in the city of Porto Alegre, Brazil.
Because the study lasted more than one year, most babies were eating various kinds of solid food and liquids in addition to breast milk.
The World Health Organization (WHO) recommends that babies are fed breast milk exclusively for the first six months of their lives, with solid foods added to the diet at that point. However, the WHO also recommends continued breastfeeding up to age two and beyond, the authors note.
For the study, the researchers checked in on babies when they were about 6, 12 and 38 months old. At six months, the study team gathered data on the number of breast milk bottles the baby drank the day before and any other liquids, like juice.
At the 12-month mark, parents reported whether they fed their babies any of 29 specific foods, including fruits, vegetables, beans, organ meat, candy chips, chocolate milk, cookies, honey, soft drinks or sweet biscuits.
Two trained dentists examined all of the babies at each of the visits.
Nearly half of the children had consumed a prepared infant formula drink by age 6 months, the researchers write in the Annals of Epidemiology, but very few still drank formula by age 1.
The researchers found that about 40 percent of children breastfed between ages 6 and 24 months had some tooth decay by the end of the study. For babies breastfed for longer than two years and frequently, that number rose to 48 percent.
"Our study does not suggest that breastfeeding causes caries," Chaffee said.
It is possible that breast milk in conjunction with excess refined sugar in modern foods may be contributing to the greater tooth decay seen in babies breastfed the longest and most often, the authors speculate in their report.
More research is needed to determine what's going on, but the findings are in keeping with professional dental guidelines that suggest avoiding on-demand breastfeeding after tooth eruption, they write.
"There are two aspects of breastfeeding - the actual human milk, which has some, but very little, ability to promote tooth decay," said William Bowen, professor emeritus in the Center for Oral Biology at the University of Rochester Medical Center in New York.
"The second is the physical aspect of breastfeeding, or even bottle-feeding, and that's where the problem arrives," he said.
Bowen was not involved in the new study.
When a baby sucks on a mother's breast or from a bottle, the baby's teeth are sealed off from saliva in the mouth. This physical barrier prevents the saliva from breaking down bacteria, and increases the chances of tooth decay, Bowen said.
Even though participants in the study came from poor backgrounds, "bad habits can form at any socioeconomic level," Bowen told Reuters Health.
About 16 percent of babies in the U.S. were still exclusively breastfed at age 6 months last year, according to the National Center for Chronic Disease Prevention and Health Promotion.
The good news, Bowen said, is that it's very easy to clean an infant's teeth.
A simple wipe in the mouth with a water-dampened cloth or Q-tip can effectively remove food before the baby's first teeth, he said, adding: "It's important to get the excess food out of the mouth."
One not-so-good habit is allowing infants to stay on a mother's nipple throughout the night, Bowen said. This usually means very little saliva circulates in the baby's mouth, which can increase the risk of decay.
The primary caregiver of the baby should also maintain good dental health because the bacteria that cause tooth decay in a baby usually come from the primary caregiver, Bowen explained.
The American Academy of Pediatric Dentistry recommends that parents take their child to its first dental visit when the first tooth appears, or no later than its first birthday.
"Finding the right age to wean a baby off breast milk can be a decision made with the support of a pediatrician," Chaffee said, adding that dental health is one consideration that could play a role.
Brushing teeth might help, Chaffee said, The study researchers collected data on tooth brushing habits, but did not investigate a specific link between cleaning teeth after the last feeding and caries.
"But anything that removes carbohydrates and sugars from the oral cavity should help prevent too decay," Chaffee said.